Gene Expression and Gene Regulation

The Link between Genes and Proteins At the beginning of the 20 th century, Garrod proposed: – Genetic disorders such as alkaptonuria result from biochemical alterations or “inborn errors of metabolism” Using a yeast, Beadle and Tatum showed that mutations: – Can produce a loss of enzyme activity – Can produce a mutant phenotype – Genes-->Proteins--> Phenotype Nobel Prize 1958

The Genetic Code Information transferred from DNA to mRNA is encoded in a set of three nucleotides (codons) Codons – Encode the information for a specific amino acid in a protein. 3 nucleotides in 1 codon Of 64 possible codons, 61 code for amino acids, and 3 are stop codons The codon AUG is a start codon and specifies methionine

The Genetic Code

9.4 The Flow of Genetic Information  Transfer of information from the linear sequence of nucleotides in DNA to the linear sequence of amino acids in a protein occurs in two steps: – Transcription – Translation

The Flow of Genetic Information (contd.) Transcription – Transfer of genetic information from the base sequence of DNA to the base sequence of RNA, mediated by RNA synthesis Translation – Conversion of information encoded in the nucleotide sequence of an mRNA molecule into the linear sequence of amino acids in a protein

What is the central The Central Dogma? DNA Transcription pre-mRNA Cell mRNA processing Cytoplasm Nucleus mRNA Translation Polypeptide

Interaction of Components Transcription and translation require the interaction of – Ribosomes, – mRNA, – tRNA, – Amino acids, – Enzymes, and – Energy sources

Interaction of Components (contd.) Ribosomes are the sites on which protein synthesis occurs tRNA molecules are adapters that recognize amino acids and the nucleotide sequence in mRNA. – Each tRNA has one anticodon (the match to a codon on a mRNA) and can only carry one type of amino acid

Transcription Begins when DNA unwinds and one strand is used as template to make a pre-mRNA molecule Has three stages: – Initiation – Elongation – Termination

ANIMATION: Transcription—A molecular view Transcription

Initiation and Termination Promoter region – The region of a gene on a DNA molecule to which RNA polymerase binds and initiates transcription Terminator Region – The nucleotide sequence at the end of a gene that signals the end of transcription

Transcription of a Gene Gene region 5’ Promoter region RNA polymerase, the enzyme that catalyzes transcription (a) RNA polymerase binds to a promoter in the DNA, along with regulatory proteins (initiation). The binding positions the polymerase near a gene in the DNA. Only one strand of DNA provides a template for transcription of mRNA.

Transcription of a Gene Newly forming RNA transcript DNA template winding up DNA template unwinding (b) The polymerase begins to move along the DNA and unwind it. As it does, it links RNA nucleotides into a strand of RNA in the order specified by the base sequence of the DNA (elongation). The DNA double helix rewinds after the polymerase passes. The structure of the “opened” DNA molecule at the transcription site is called a transcription bubble, after its appearance.

Transcription of a Gene Transcription site Growing RNA transcript G (c) What happened in the gene region? RNA polymerase catalyzed the covalent bonding of many nucleotides to one another to form an RNA strand. The base sequence of the new RNA strand is complementary to the base sequence of its DNA template––a copy of the gene. A A G

Pre-mRNA processing Transcription produces large mRNA precursor molecules called pre-mRNA Pre-mRNA is processed in the nucleus to produce mature mRNA

Processing and Splicing mRNA Cap – A modified base (guanine nucleotide) attached to the 5’ end of eukaryotic mRNA molecules Poly-A tail – A series of A nucleotides added to the 3’end of mRNA molecules

Processing and Splicing mRNA Introns – DNA sequences present in some genes that are transcribed, but are removed during processing and therefore are not present in mature mRNA Exons – DNA sequences that are transcribed, joined with other exons during mRNA processing, and translated into the amino acid sequence of a protein

Processing and Splicing of mRNA Unit of transcription in DNA strand ExonIntronExonIntronExon Transcription into pre-mRNA Cap Poly-A tail Snipped out Mature mRNA transcript

Alternative Splicing Smooth-muscle mRNA Exons 1–12 pre-mRNA Striated-muscle mRNA

Mutations in Splicing Sites Splicing defects cause several human genetic disorders One hemoglobin disorder,  -thalassemia, is due to mutations at the exon/intron region and lower splicing efficiency